This invention relates to thermally actuated cut-off links (also referred to commonly as thermal fuses or cut-offs) and normally open switches of a type which responds to the ambient temperature when it reaches a given value. Normally closed versions of such thermally actuated cut-off links, for examples, are frequently physically incorporated into the windings of electric motors and in other devices requiring thermal protection and electrically connected in series with such devices so that the cut-off link will de-energize the device involved when the ambient temperature exceeds a given safe value.
Ambient thermally actuated cut-off links have been manufactured in two different configurations, one of which is disclosed, for example, in U.S. Pat. No. 3,180,958 to P. E. Merrill, and the other of which is disclosed in U.S. Pat. No. 3,944,960 to Audette et al. In both of these types of cut-off links the ambient heat is transmitted to the interior of the link through a generally elongated cylindrically-shaped conductive casing initially substantially closed at one end and open at the other end. A first power lead extends longitudinally into an insulating closure in the open end of the housing and terminates in a flat end making a separable contact interface with a spring metal connector member spring-urged thereagainst and having a plurality of contact-forming arms resiliently pressing against and making sliding contact with the conductive interior walls of the casing. A second power lead extends longitudinally into the closed end of the casing where it is crimped to or otherwise connected to the end wall of the casing to make a permanent inseparable low resistance engagement with the end wall. The interface of the contact-forming arms of the connector and the inside walls of the casing and the interface of the first power lead and the connector form two separable electric contacts between the power leads having a resistance much greater than that between the second power lead and the casing end wall. It is believed that at high rated currents of large electric motors or other devices requiring thermal protection heat develops at these separable contact interfaces which can appreciably affect the ambient temperature at which the link opens, which is lowered thereby.
In the type of thermally actuated cut-off link exemplified by the Merrill patent, the casing contains a sandwich of elements including a pellet of meltable material at the closed end of the casing, a first partially compressed spring, the contact-forming arm carrying connector urged against the end of the power lead passing through the open insulated end of the casing, and a second weaker partially compressed spring on the opposite side of the connector which applies a force to the connector in a direction tending to move the connector away from the power lead. When the pellet melts at the control temperature, the stronger spring expands until its force equals that of the weaker spring, and then the originally weaker spring expands to push the connector away from the end of the adjacent power lead to open the cut-off link.
In the type of ambient thermally actuated cut-off links exemplified in the Audette et al patent, where deformable contacts are separated from an adjacent contact surface by an arm-deforming member (in a manner like that disclosed in an earlier U.S. Pat. No. 3,274,363 to McGirr et al and in a later U.S. Pat. No. 4,109,229 to Plasko), the sandwich of elements within the casing includes only a single partially compressed spring. This spring applies pressure against a meltable pellet, in turn, positioned contiguous to an arm-deforming member which, when the pellet melts, is pushed against the contact-forming arms of the connector to deform the arms inwardly away from the interior of the casing to open the fuse. In the types of cut-off links exemplified by the Merrill, Audette et al, and Plasko links described above, the constructions involved are such that the resistance of the contact interfaces described cannot be adjusted during or after assembly thereof, and differences in the internal resistance of what appear to be identical cut-off links, can cause variations in the ambient temperature at which identical appearing fuses open. Also, the deformable contact of these cut-off links are made of spring metal and the contact resistances between the contact-forming arms and the adjacent contact surfaces are much higher than desired and can increase undesirably with ambient temperature and current generated heat at the contact interface. A premature opening of the cut-off links below the rated temperature due to the creeping of the pellets is also believed to be a particular problem with the Audette et al fuse since such creeping can apply a contact-opening force on the contact-forming arms because of the location of the pellet adjacent to the arms.
An object of my invention is to provide an ambient thermally actuated cut-off link design of the type having a cylindrical casing and contact-forming arms engaging the casing to provide continuity between the power leads of the cut-off link, and which is constructed in a manner where the internal resistance of the cut-off link can be substantially reduced from the heretofore obtained in cut-off links of this type, thereby minimizing the internally generated heat of the link.
Another object of my invention is to provide an ambient thermally actuated cut-off link and a method of making the same where the control temperature thereof can be held closer tolerances, especially with higher currents, than can the thermally actuated cut-off links described above. A related object of the invention is to provide a unique thermally actuated cut-off link and a method of making the same wherein the internal resistance thereof is readily adjusted to a desired low value during assembly of the cut-off link and wherein this adjusted resistance remains at or close to the adjusted value for the life of the cut-off link.
Still another object of the invention is to provide an improved reliable ambient thermally actuated cut-off link or switches which can be mass produced at a relatively low cost.
Another disadvantage of the thermally-actuated cut-off link disclosed in the Audette et al patent is due to the location of the meltable pellet between the arm-deforming member and the connector contact-deforming arms. This causes the unfortunate result that, when the pellet melts, the initial expansion of the spring is used to move the arm-deforming member a distance equal approximately to the initial thickness of the pellet, before the force of the spring is effective in deforming the contact-forming arms, so that the initial greater force of the spring required causes pellet creep rather than opening of the link. A larger spring is thus necessary to produce the force necessary to deform the contact-deforming arms engaging the casing walls which will aggravate the problem of creep. Accordingly, another object of my invention is to provide an ambient thermally actuated cut-off link of the type exemplified by the Audette et al cut-off link wherein an arm-deforming member is placed under spring pressure and upon the melting of the fusible pellet the expansion of the spring which urges the arm-deforming member toward the connector arms is immediately operative to deform the connector arms.